This invention relates to digital signal information demodulation and more particularly this invention relates to demodulation of packet-based wireless signals in the presence of interference from one or more of a known class of packet-based signals. Current narrowband Time Division Multiple Access (TDMA) cellular systems, such as IS-54 or GSM, are designed to accommodate several time-multiplexed users in the same RF channel, thus providing a capacity improvement over older analog Advanced Mobile Phone Service (AMPS) Frequency Division Multiple Access (FDMA) systems. This capacity is limited by frequency reuse, which is related to a receiver's ability to tolerate Co-Channel Interference (CCI), especially with cell splitting and cell reduction as in Personal Communication Service (PCS) systems.
FIG. 1 is a diagram of a cellular network utilizing frequency reuse. The figure shows a “7/21” reuse pattern, corresponding to a reuse factor of 7, with 120 degrees sectoring, which is know to those skilled in the art. In cells involving frequency reuse, the receiver must detect and receive a desired signal from an Intended User (IU) in spite of CCI signals that share the same RF channel. The presence of CCI signals may degrade the Bit Error Rate (BER) associated with reception of the desired signal by orders of magnitude, even when the CCI power is significantly below that of the IU. Furthermore, multipath-induced Intersymbol Interference (ISI) in frequency selective fading channels constitutes another major source of degradation. Hence, CCI and ISI mitigation capabilities of a receiver can lead to increased capacity and better overall system performance.
Existing CCI mitigation techniques attempt to suppress the contribution from the interfering signals to the received waveform. One class of such techniques employs antenna arrays that can introduce spatial nulls in the direction of the CCI and enhance the IU(s). The applicability of these techniques, however, is constrained by the number of antenna elements. (Typically, the number of beams or nulls formed cannot exceed the number of elements.) Similarly, there are user/CCI geometries for which linear combining techniques necessarily fail (e.g., when an interferer and the IU signals lie in regions where a linear combiner has equal gain). Another class of CCI mitigation techniques uses traditional Decision-Feedback Equalizers (DFE) that exploit the cyclo-stationary nature of CCI in order to reduce its effect in the detection of the IU. These techniques are effective in cases when the IU and the CCI signals are separated in some dimension (e.g., space or time). However, they are prone to severe performance degradation when there is little or no separation. Furthermore, even in cases of good separation, suppression of strong CCI signals can still result in significant reduction of the received Signal-to-Noise Ratio (SNR) for the IU.
Joint detection techniques provide an optimal approach to data detection in the presence of ISI and CCI. In contrast to the traditional approaches described above, joint detection techniques do not attempt to null the CCI signals; they model and detect CCI signals along with the desired signal from the IU. Detection of CCI signals is introduced as a way of assisting the detection of the IU, not as a goal of its own.
One straight-forward method using joint detection techniques is disclosed in U.S. Pat. No. 6,249,518B1 entitled TDMA SINGLE ANTENNA CO-CHANNEL INTERFERENCE CANCELLATION issued Jun. 19, 2001 to Jian Cui and assigned to Nortel Networks. This patent describes an approach to cancellation of co-channel interference from TDMA packet-based signals. The approach assumes a simplified channel model wherein the channel is assumed to be memoryless so that decision feedback does not need to take into account pre-existing states of the desired signal or of the interference. Hence, the system uses a multi-stage block decision feedback channel gain estimation algorithm and a memoryless joint detection algorithm. However, since the memoryless model breaks down in systems with a transmission bandwidth larger than the coherence bandwidth of the channel, the receiver fails to provide significant mitigation of co-channel interference and totally fails to provide any mitigation of intersymbol interference.
Another approach to cancellation of co-channel interference from TDMA packet-based signals is disclosed in U.S. Pat. No. 5,995,499 entitled SIGNAL DETECTION IN A TDMA SYSTEM issued Nov. 30, 1999 to Hottinen et al. and assigned to Nokia Telecommunications Oy.
The following references selected from the paper forming the basis of the priority provisional application, namely, G. Paparisto, P. Panagiotou, and K. M. Chugg, “A Single Packet Method for Adaptive Maximum Likelihood CCI Identification and Mitigation,” published Proc. of IEEE Globecom Conference, Rio de Janiero, Brazil, Dec. 8, 1999, represent background and the general state of the art:
J. Lin, F. Ling, and J. Proakis, “Joint data and channel estimation for TDMA mobile channels,” in Proc. PIMRC '92, October 1992, pp. 235–239. This paper describes PSP for ISI mitigation.
N. W. K. Lo, D. D. Falconer, and A. U. H. Sheikh, “Adaptive equalization for co-channel interference in a multipath fading environment,” IEEE Trans. Commun., vol. 43, pp. 1441–1453, February/March/April 1995. This paper describes an alternative co-channel interference mitigation algorithm.
S. Verdu, “Minimum probability of error for asynchronous Gaussian multiple-access channels,” IEEE Trans. Inform. Theory, vol. IT-32, pp. 85–96, January 1986. This paper is the seminal paper on joint modeling of interference.
R. A. Iltis, “A digital receiver for demodulation of CDMA waveforms with a-priori unknown delays and amplitudes,” Proc. MILCOM'91, 1991, pp. 5.3.1–5.3.4. This paper describes a PSP-based interference mitigation algorithm for CDMA waveforms.
K. Giridhar, S. Chari, J. J. Shynk, and R. P. Gooch, “Joint demodulation of co-channel signals using MLSE and MAPSD algorithms,” Proc. ICASSP '93, April 1993, vol. 4, pp. 160–163. This paper describes a technique for demodulation of co-channel signals.
S. N. Diggavi, B. C. Ng, and A. Paulraj, “Joint channel-data estimation with interference cancellation,” Proc. ICC '98, Atlanta, Ga. June 1998, pp. 465–469. This paper describes a technique for estimating joint channels and data.
R. Raheli, A. Polydoros, and C.-K. Tzou, “Per-Survivor Processing: A general approach to MLSE in uncertain environments,” IEEE Trans. Commun., vol. 43, pp. 354–364, February/March/April 1995. This is the seminal paper on per-survivor processing.
G. Paparisto and K. M. Chugg, “PSP array processing for multipath fading channels,” IEEE Trans. Commun., vol. 47, pp. 504–507, April 1999. This paper describes PSP for ISI mitigation based on antenna array measurements.
J. Rissanen, “Modeling by shortest data description,” Automatica, vol. 14, pp. 465–471, 1978. This paper describes minimum description length criteria.
It should be noted that adaptive detection using per-survivor processing is known. One approach is described in U.S. Pat. No. 5,432,821 issued Jul. 11, 1995 of Andreas Polydoros and Riccardo Raheli.
What is needed is an interference detection scheme which provides a more accurate model of a signal channel containing interference from identifiable sources.